NEW YORK — Ulrich Franzen, a German-born architect whose fortresslike buildings seemed to buttress the psychological landscape of New York City during the shaky 1970s, and who gave it some buoyance, too, with skywalks, died Oct. 6 in Santa Fe. He was 91.

Not everyone liked the skywalks, which connect buildings Mr. Franzen designed at Hunter College on Lexington Avenue. Neighbors lamented the loss of sunlight. But Mr. Franzen, a Modernist subscriber to the form-follows-function credo, considered them the functional equivalent of ivy-covered walkways for urban students.

DAVID FRANZEN

Mr. Franzen’s multicategorical research tower at Cornell University.

It would ‘‘become the college community’s main street,’’ he wrote of the skywalk plan in 1972 in the college’s student newspaper, ‘‘well above rush-hour traffic at street level.’’

Mr. Franzen was part of a generation of prominent American architects, including Philip Johnson, Paul Rudolph, and I.M. Pei, to emerge from the Harvard School of Design after World War II. The group was deeply influenced by the Bauhaus architecture masters Walter Gropius and Marcel Breuer, who taught at Harvard after fleeing the rise of Nazism in Germany in the 1930s.

Mr. Franzen’s early works are exemplars of the Modernist style, among them his first designs for residences, most of them clean-lined, single-level structures, wrapped in sliding glass and flooded with light.

He also helped design the first-generation shopping mall Roosevelt Field, on Long Island, working with Pei and Henry N. Cobb, another graduate of the Harvard program.

Mr. Franzen’s first major solo project was the fortresslike Alley Theater in Houston, which opened in 1968 for the city’s resident theater company. Critics praised it as a triumph of the Brutalist style, characterized by the use of rough exterior materials like concrete. But many Houston residents hated it. One letter in The Houston Chronicle condemned the building’s ‘‘totalitarian’’ aura.

Mr. Franzen applied Brutalism again in designing two 17-story concrete and glass towers for Hunter College, a branch of the City University of New York.

Begun in 1972 and finished in 1984 the buildings stand on opposite sides of Lexington Avenue between 67th and 68th streets. Many residents were surprised by the skywalks, connecting the buildings at the third and eighth floors. A third walkway links buildings over 68th Street.

‘‘It’s the one at the third-story level that’s most oppressive, I think, because it’s low,’’ Margot Wellington, executive director of the Municipal Art Society, said in an interview for the 2006 book ‘‘New York 2000: Architecture and Urbanism Between the Bicentennial and the Millennium,’’ by Robert A.M. Stern. ‘‘The two together,’’ she said, ‘‘take out more sky than you think.’’

But the walkways became central arteries of college life. Stern, also dean of the Yale School of Architecture, said Wednesday that people had grown more comfortable with his work at Hunter College.

‘‘It wasn’t everybody’s idea of a charming college campus experience, but it was interesting,’’ he said. “It widened the public perception about urban space.’’

Mr. Franzen’s most high-profile project in New York was the world headquarters for the Philip Morris Cos., across from Grand Central Terminal at Park Avenue and 42d Street. The tower, begun in the late 1970s and completed in 1982, carried symbolic importance.

“It was about the city’s viability as a corporate center,’’ said William J. Higgins, a landmarks preservation consultant.

‘‘He built a concrete fortress,’’ Higgins said, ‘‘that sort of summed up the emotional landscape of New York in the ‘70s.’’

Ulrich Joseph Franzen was born in Duesseldorf, Germany.

After receiving his bachelor’s degree at Williams College in Massachusetts, he spent a semester at Harvard’s architecture school before joining the Army and serving in the Office of Strategic Services. He returned to Harvard and received his master’s degree in 1950.

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